In liquid chromatography, especially in high performance liquid chromatography (HPLC), substances are separated by being flushed through a chromatography column at high pressure with the aid of a solvent. To produce the liquid flow required for this purpose, there is a need for pumps capable of continuously supplying what is usually a very low, constant flow rate at high pressure. In order to achieve more rapid separation or separation with better resolution in HPLC, ever higher pressures have been used in recent years.
To produce the liquid pressure, piston pumps are usually used in HPLC. These pumps generally employ passive check valves at the inlet and at the outlet, these valves usually being designed as ball valves. Check valves of this kind switch from a CLOSED state to an OPEN state and vice versa in accordance with the direction of the applied pressure difference of the fluid flow to be controlled, said difference being applied across the valve (or the switching element concerned), or in accordance with the direction of the fluid flow through the through flow channel of the valve. The valves each contain a valve seat, a switching element designed as a ball and a guide element for guiding the switching movement of the switching element. The valve seat element and the switching element must each consist of a very hard and yet tough material in order to allow a sufficiently high surface pressure in the CLOSED state for the high pressures to be controlled. The switching element designed as a ball often consists of ruby and the valve seat often consists of sapphire. However, ceramic materials, such as AlO2 or ZrO2, are also suitable for producing these elements, for example. These materials have the advantage that they are very hard and strong in compression. This is necessary because extremely high pressures occur locally in the region of the sealing surface, i.e. the contact surface between the valve seat element and the switching element, and these can amount to a multiple of the liquid pressure.
In order to increase the reliability of a check valve of this kind, two (or even more) full valves or valve units arranged in a common housing can be connected in series to form a dual check valve. For correct operation, it is therefore sufficient if one of the two valves or one of the two valve units is still intact, in particular leaktight in the CLOSED state.
For correct operation of a check valve, it is necessary to seal off the elements of a valve unit, in particular, the valve seat element and the switching element guided movably in a guide element, in such a way that no liquid can pass through the check valve or the valve unit concerned in the CLOSED state due to leaks. For this purpose, the individual components of the valve must be sealed off relative to one another and relative to the housing accommodating them. This sealing must withstand alternating loads up to the maximum pressure of the pump on a sustained basis.
Ruby, sapphire and ceramic are very strong in compression but withstand only a relatively low tensile stress. Since the valve seat element and also the guide element for the switching element generally have a (usually axial) opening for the passage of the fluid flow to be controlled and, consequently, the fluid pressure acts on the inner walls of said elements, (primarily azimuthal or tangential) tensile stresses arise during a deformation of the walls of said elements, possibly leading to fracture of the material. The pressure of the ball on the valve seat, which is formed by a switching element or ball contact surface extending obliquely to the axis of the through opening for the fluid, can also lead to fracture of the seat element due to the resulting (predominantly azimuthal) tensile stresses.
In known check valves, there are various known possibilities for sealing off the components relative to one another and relative to the housing (in other words for sealing off the through flow channel defined by the components).
In general, all the components of a valve unit, in particular the valve seat element and the guide element together with the switching element accommodated therein, are installed in a metal sleeve. Caps made of PEEK are used at the two ends of the sleeve in order to seal off the entire unit. It is also possible for two valve units to be arranged in the metal sleeve and, in this case, the use of a further, thin sealing washer made of PEEK between the two valve units of the dual check valve is known. This design is reliable up to pressures of about 1000 bar. At higher pressures, however, there is the problem that the caps are subject to excessive plastic deformation in continuous use and begin to leak over time.
Another possibility is to use thin PEEK washers or metal washers at the ends as well, as is the case with the valves illustrated in EP 1 514 027 B1. However, only sealing at the ends is possible with such washers, and this requires a complex design in the case of a dual ball valve. Moreover, high demands are made of the quality and structure of the surface. In EP 1 514 027 B1, for example, concentric grooves are proposed.
For the problem of the limited stability of the seat and the guide element due to the occurrence of tensile stresses, one known practice is to make the contact surface between the valve seat and the guide element for the ball with a controlled leakage in order to allow a pressure equalization between the inside and the outside. The sealing between the two individual valves of the dual ball valve must then be such that this region is also leaktight with respect to the sleeve.
Another known practice is to press fit the valve seat elements into a metal ring and thereby subject them to a radially inward load. The disadvantage of this variant consists in the additional production outlay associated therewith.
Finally, there is the possibility of producing the guide element for the switching element, not from ceramic, but from stainless steel, which has a higher tensile strength. However, the stability problem of the valve seat element cannot be solved in this way since stainless steel is not a suitable material for the valve seat (or for the switching element).
U.S. 2011/0094954 A1 discloses the practice of making the valve seats conical at the outer circumference and preloading them in the axial direction by means of complementary mating parts. A preload with a radially inward component is thereby also produced, and this is capable of absorbing some of the expansion of the annular seat element caused by the fluid pressure. However, this variant requires an increased production outlay and an increased number of components with complex contours, especially in the case of a dual ball valve.